[ALSA] PCM - clean up snd_pcm_lib_read/write

Introduce a common helper function for snd_pcm_lib_read and snd_pcm_lib_write
for cleaning up the code.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>